1. Field of the Invention
The present invention relates to personal computers, and more particularly to a personal computer having random access memory which is functional immediately upon power up for use in system initialization.
2. Background of the Invention
Personal computers are in common use in offices and homes throughout the world. The capabilities of these machines have progressed dramatically because of increased microprocessor speed and reduced cost of random access memory, RAM. Computers with hundreds of megabytes of RAM are available to consumers at reasonable prices. The most common type of RAM now in use is dynamic random access memory, DRAM. While DRAM designs make it possible to have large fast memory units at reasonable cost, the DRAM chips must be initialized each time the computer is powered up. Before initialization, the DRAM chips will not function as memory, i.e., it is not possible to store information in, or retrieve information from, the DRAM.
The need for DRAM initialization has created a problem in personal computers at the system level. The code which initializes the DRAM is normally part of the basic input/output system, BIOS, which is stored in the system read only memory, ROM. The BIOS is what starts or boots up personal computers when power is turned on. The DRAM initialization code must be written to avoid accessing any DRAM memory locations, because during initialization the DRAM is not functional as memory. However, all computer code, including the DRAM initialization code in the BIOS, needs some RAM in which to store variables while it operates. Without some RAM available, the code will not work.
For current designs, the only RAM type memory available to the BIOS upon power up is the processor's own register files. These register files however, only provide storage for about fifty bytes. This small available storage severely limits the programmers who write the initialization code and results in awkward code. For example, nested subroutines cannot be invoked because no memory is available to hold their return addresses. Limitations are also placed on the amount of system checking which can be done. For example it is not possible to check whether all DRAM memories are properly populated and certain assumptions are normally made. It would be better to completely check the memories, but without sufficient memory available, this simply is not possible.